Socket Mode support

[seratch]

This pull request is for Socket Mode support #159. The details of Socket Mode is not yet publicly shared. As it's still in beta, the implementation on this SDK side can be changed until the GA release (v1.2.0) I used to use #160 for Socket Mode development, but we will continue with feat-socket-mode branch in this repository from now on.

Remaining TODOs are:

• Add relevant unit tests
• Add comments in code
• Add documents at the timing of GA release

Category (place an x in each of the [ ])

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements (place an x in each [ ])

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov-io]

Codecov Report

Merging #176 (5670cdb) into main (47063fa) will decrease coverage by 0.51%.
The diff coverage is 82.15%.

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
- Coverage   92.12%   91.61%   -0.52%     
==========================================
  Files         149      159      +10     
  Lines        4508     4744     +236     
==========================================
+ Hits         4153     4346     +193     
- Misses        355      398      +43     

Impacted Files	Coverage Δ
slack_bolt/adapter/socket_mode/async_handler.py	0.00% <0.00%> (ø)
slack_bolt/lazy_listener/async_internals.py	100.00% <ø> (ø)
slack_bolt/lazy_listener/internals.py	86.66% <ø> (ø)
slack_bolt/listener/async_listener.py	98.41% <ø> (ø)
slack_bolt/listener/custom_listener.py	100.00% <ø> (ø)
slack_bolt/listener_matcher/async_builtins.py	100.00% <ø> (ø)
...ck_bolt/listener_matcher/async_listener_matcher.py	96.55% <ø> (ø)
slack_bolt/listener_matcher/builtins.py	93.53% <ø> (ø)
...k_bolt/listener_matcher/custom_listener_matcher.py	100.00% <ø> (ø)
slack_bolt/middleware/async_custom_middleware.py	96.00% <ø> (ø)
... and 25 more

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[seratch]

I've already verified the implementation has been stably working with Socket Mode connections for weeks but this PR still lacks unit tests for the new Socket Mode adapter. I will add some tests in this PR by the end of this week.